netfilter: nft_hash: fix hash overflow validation

The overflow validation in the init() function establishes that the
maximum value that the hash could reach is less than U32_MAX, which is
likely to be true.

The fix detects the overflow when the maximum hash value is less than
the offset itself.

Fixes: 70ca767e ("netfilter: nft_hash: Add hash offset value")
Reported-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_hash.c

@@ -76,7 +76,7 @@ static int nft_hash_init(const struct nft_ctx *ctx,
 	if (priv->modulus <= 1)
 		return -ERANGE;
 
-	if (priv->offset + priv->modulus - 1 < U32_MAX)
+	if (priv->offset + priv->modulus - 1 < priv->offset)
 		return -EOVERFLOW;
 
 	priv->seed = ntohl(nla_get_be32(tb[NFTA_HASH_SEED]));